By Wikipedia's definition, A Change of Seasons and Nil Recurring are albums. I think it's largely up to the artist, and what they consider it to be.
To me, 'Studio Album' implies that it's mostly or entirely original material. A cover album, even if it's recorded in a studio, isn't a 'studio album' in that sense. Yes, technically it is, but when most people refer to a studio album, they're talking about original music by that band or artist.
Even if Feedback isn't counted as an EP (which it is), Clockwork Angels is Rush's 19th album.